AP Chemistry Exam Reflections 2020

Kristen Drury | Tue, 07/28/2020 - 10:41

pen on notebook with text: AP Test Reflection 2020

The 2020 AP Exams were definitely challenging between switching to remote instruction, removing units of instruction, implementing an online exam, and reformatting the exam, among other issues. All these changes led to anxious students and teachers. One of the anxieties that lingered past the exam was how it would be read and scored. We worried that the handwriting would be too hard to read, with shaky pictures and pages out of order. As a reader, this was a lot less of an issue than I had imagined it would be. I actually liked seeing the pictures of handwritten answers more than typed answers because it seemed that the handwritten answers were more well thought out and explained, whereas the typed answers were often too short, without work shown. We were able to rotate pictures and zoom in and out of the pictures, which helped out a lot. While training and scoring three separate exams (we ran out of the questions I was assigned to and I was switched to help out on other questions), I kept notes about some common issues I saw as I scored.

The test included a lot of questions in which students had to agree or disagree with a statement with evidence and reasoning. Very often students would agree with a statement and support it with evidence and reasoning that actually disagreed with the statement. At least in the scoring guidelines I used, it mattered less which word the student used (agree/disagree) and more about what evidence and reasoning they used. In other words, if the answer should have been disagree, and the student wrote “agree” but then followed up their choice with appropriate evidence and reasoning that actually shows they in fact disagreed, we graded that as the student corrected themselves as they wrote. That is great news for the student since the answer is scored in their favor; however, this is definitely something we should work on in our classes. I would recommend trying to add questions like these to your formative assessments.

Along these lines, with any exam, students have failed to read the full question and answer with information to prove they understand the material. Very often the students only restated the prompt with out backing up their claim. Practicing skills such as writing Claim, Evidence, Reasoning statements through out the year will help students become more thoughtful when providing answers to questions. The structure helps them identify the need to back up their claims not only with information from the stem or prompt of the question, but reasoning from what they have learned through out their time in your class. Sometimes that reasoning can be in the form of mathematical work. I would suggest that students show all of their work, even if it is a simple subtraction or addition, because it will show the reader where the students got their numbers. This year, since the exam was online, technically students could be taking the test right next to one another, so many of the questions required work shown, even for simple calculations. Showing all work is something to practice throughout the year in order to have the students used to showing all work as a habit by May. I often say the short one word or calculated answer is their claim, the evidence is the numbers they are grabbing from the question and reference tables, and the rest of the calculation is the reasoning for these math examples. In the event that the numbers are coming from charts and graphs, it is especially important for the students to record the numbers that they deemed most important for the calculation. For example, when reading a heating curve to calculate the energy during heating, if the student is asked for the temperature change, they should not just record their final answer. A better answer would clearly show the final and initial temperatures used to determine the temperature change.

When a question asks students to explain an error in the lab, all too often the students just mention they agree the error in the lab will change the result, or they disagree that the error will not change the result. This is simply not enough. Encourage students to “trace” the error through a calculation, whether they prefer to do that narratively or recalculate with a skewed number. A narrative answer would require the student to explain how the error affects specific calculations and whether the final answer would be larger or smaller than the actual answer. But a mathematical representation is just as valuable in the eyes of the reader, and sometimes preferred. And, units are important! In many educational programs, such as Modeling Instruction, and POGIL, students are encouraged to keep their numbers with their units through out the calculation for multiple reasons. One is to better understand where the final answers’ units are coming from. But another reason is just to help remember to write them at all. Questions from topics such as thermodynamics, kinetics, and light equations are particular known for requiring units in the answer.

The rest of my notes have to do with specific topics I encountered in my reading experience and some thoughts I had about tweaking instruction for net year.

Unit		Ideas
1	Atomic Structure and Properties	Periodic trends should be explained in terms of protons attracting closer or further and more or less electrons. Not placement on the periodic table. Answers that only compare “how far to the left” an element is or state reasoning such as “atomic radius decreases down a group” without mentioning proton and electron attraction will not be awarded points. Malleable is due to a sea of electrons allowing ions to move about. Compare that definition to the density and ductility of metals. There were questions that called out malleability, and it appeared that students learned it means the metal was moldable but not necessarily what the cause was.
2	Molecular and Ionic Compounds Structure and Properties	Emphasize ionic compounds with diatomic like CaCl₂ have only ionic bonds and character because the chlorine molecule will break before bonding to the calcium. Many students answered that compounds like calcium chloride had both ionic and covalent bonds: ionic between calcium and one chlorine atom and covalent between the two chlorine atoms. In coulombs law, the “r” is not the radii of the particles but rather the distance between the two particles. Many students compared the Lattice Energies of two compounds based solely on atomic radii of the ions rather than continuing to describe that when the radii of the ions are smaller, the distance between the nucleus of one ion and the electron cloud of the second ion is closer and has a stronger attraction.
3	Intermolecular Forces and Properties	This is a bit of a picky subject that I value, but one molecule of a compound does not have hydrogen bonds. A lot of students phrase their answers in a way that shows this misconception is common. Hydrogen bonds exist between molecules that are themselves, covalently bonded. So if prompted to answer why one compound has stronger intermolecular forces than another, a student may answer “because it has hydrogen bonds between H and F, O, or N atoms” and then point to a covalent bond. A better answer is “Hydrogen bonds are present between and an electropositive H (covalently bonded to a F, O or N) and the (electronegative) F, O or N of a neighboring molecule.” London Dispersion Forces depends on the electron cloud and its polarizability, not size of molecule. Although the two may be correlated, the answer should be in terms of the attraction forces due to the polarizable electron cloud.
4	Chemical Reactions	Perform more dilutions with random solutions. Lots of students just assume everything being diluted is acidic or basic and then actually explain a dilution as if it were a titration! When safe and appropriate, have students dilute their own various solutions so they have more experience with it.
5	Kinetics	Practice more graphing of zero, first and second order reactions. Identify that the graph with the linear slope is correlated to that specific order and rely less on if the slope is positive or negative. In other words, it is more important that a second order graph is linear when graphing the inverse of a concentration versus time than that the slope was positive.
6	Thermodynamics	“Bonds broken minus bonds formed” is an algorithmic approach to bond energy calculations that may lead to many errors in calculations but also misconceptions when trying to explain endothermic and exothermic reactions in terms of bond energy. Instead, I try to have them concentrate more on breaking bonds requires energy and forming bonds releases energy, therefore negating the values of bonds formed. Without giving the algorithmic (bonds broken minus bonds formed) students must have a better conceptual understanding because they cannot rely on that calculation and their explanations with reasoning tend to be clearer.
7	Equilibrium	Giving the name of the chemical principle is not enough; the principle must be explained. This is listed here because I most often saw this issue when asked to discuss shifts in equilibrium. Students would write, “The reaction shifts left due to LeChatelier’s Principle.” A better answer would mention what species collided more often to form what new species. Acids and bases’ molarity are not the only things that can be identified via titration. I would encourage you to decouple acidic and basic solutions from the techniques of dilutions and titrations as much as possible. View dilution and titration as techniques for multiple solutions such as redox reaction as well. When asked how to perform or prepare a titration or dilution, the students confuse the two often because they think they are both only for acids and bases. Maybe, if we decouple the acids and bases from the technique, they can answer the questions more objectively, rather than going into autopilot. An example was when a student was asked to set up a dilution of a neutral salt and the student answered by describing adding a base to neutralize it. This was very common. K_eq values never add. Reinforce that these are product expressions. By the end of the topic, students often seem to forget how K_eq expressions were derived and just create them out of habit, not understanding, which could be why when a question asks students to combine reactions to find a new K_eq value for the combined reaction, students add the K_eq values instead of multiplying. I recommend revisiting writing the K_eq expressions for both elementary reactions and how it relates to the combined K_eq to show the connection again. Evaporation of water at equilibrium will not change concentration of ions because as water evaporates the ions in solution will precipitate out to maintain equilibrium. Therefore, the number of moles of dissolved ions in relationship to the liters of solution is still in proportion to one another. Many students mentioned that evaporating water would cause the concentration of ions to increase due to the same moles dissolve in less water. And other students mentioned that the concentration would remain the same but they couldn’t explain themselves.

In addition to my ideas, Paul Bonvallet, the AP Chemistry Exam Chief Reader, provided his “Top 10 Suggestions for Students and Teachers of AP Chemistry” during a webinar hosted by AP Teach (www.apteach.org).

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I hope that some of these reflections will help you plan for next year. If you are an exam reader and would like to share your reflections, please comment your ideas on this post! And if you are a chemistry teacher with questions or additional ideas, we would also love to hear from you in the comments section! We are in this together, and we become better together.

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